Help with motor trailer design please

[cirrus18]

I am a keen cyclist who is getting on in age and now I require a bit of help up those hills etc. I have a lightweight Specialized Sirrus with carbon front forks and nine speed Shimano gears.
I don't want to modify the bike and want to keep it as it is and as light as possible.
My thoughts are for a powered trailer.
I only want occasional assistance and want to pedal as much as I can with minimal drag from the motor. To this end I thought that the Tongxin would be a good bet, as it offers low resistance when the motor is not in use.
Unfortunately, they don't seem to be readily available to purchase without paying an exorbitant price.

I have been searching the Internet and came across these geared motors 1016Z and 1018Z__¡ËSHENZHEN UNITE INDUSTRIES CO.,LTD¡Ë I thought one of these, mounted on a trailer, driving a 16 inch rear wheel type with a cog and free wheel, chain driven, would be the answer.
The 250 W motors are quite light at just over 2 kg.

Having a free wheel would mean that there should be no drag when the motor is not in use.

The trouble is I don't know how these motors are designed to be used, which one would be best and what size cog would be required on the wheel, for say 20 mph top speed.
If anybody out there could give me some help on designing this trailer, or other suitable motor suggestions, their help would be most appreciated.

 

[flecc]

Those Shenzen Unite motors are the ones that are used on the Currie Electrodrive kit and bikes, and they are used with a small reduction chain drive onto a 26" rear wheel hub, sitting alongside the rear wheel.

On the website below you'll see photos of it on bikes of various kinds and also a two wheel trailer that uses the motor kit onto it's offside 20" wheel. The kit on a 16" wheel would only give around 11 mph. Of course if you rigged up your own chain drive from the motor shaft you could gear it as you wish.

Electrodrive site

With this kind of side-of-rear-wheel motor freewheel, there is no unpowered drag as you say. With the types of hub motor that are built into a wheel and which have internal epicyclic gear reduction, there is the drag of the epicyclic gears since they are engaged with the hub all the time whether under power or not. Their freewheels are between the motor and the gears, not between the gears and the hub.

I guess you are thinking of a Yak style one wheel powered trailer, and some have already been created like that, like this one by The Dutch Bike Company, here on a recumbent trike but intended for all types of bike. the lower half of the shell holds the motor drive and battery, the upper half is for luggage:

 

[Beeping-Sleauty]

Yak eMotor

Liked this one...

 

[Footie]

For information and ideas you could try the following sites:

Bidwell Bike Pusher:* A Removeable Power Source for Your Bicycle..

This one could be modified with a motorised wheel but not sure if the clips would hold it on
Extrawheel - one-wheel bicycle trailer : bike trailer and panniers

Projects on electric cars, etc (food for thought?)
Built For Fun Electric Vehicles - Plans, Information and Great Resources for DIY'ers

Trailers
Luggage & Load Carrying : Transportation Trailers AMBA Marketing

Carry Freedom - Award winning UK bicycle trailer maker.

Tony's Trailers -- Bicycle Trailers

Finally (just to add another idea) .... not done it yet but I have been thinking about making my own powered trailer by using a modified tag-a-long

tag a long, Sporting Goods, Crafts, Clothes, Shoes Accessories items at low prices on eBay.co.uk

-----------------------------------------------
Cougar Mountain Electric Bike 36v 200w rear wheel Hub motor (Jul07)
fitted with 10 Ah LiFePo4 battery (Apr08)
Maximum range (road/hills - Cornwall) 18 miles
Maximum range (on flat) 25 miles

 

[cirrus18]

Many thanks for the replies they have made interesting reading.
I have decided on a mono wheel trailer and was wondering if anybody knew where to get a suitable tow hitch from. One designed for a mono trailer i.e. connected to both sides of the bike frame.

Looking at the specs off suitable motors they seem to come with a 7 or 11 tooth cogs (what size would be best to you think) and rotate around 2000 rpm.
I have been looking for a online calculator or formula to work out what size cog to fit on the wheel, without success.
So if the motor is going at 2000 rpm with say a 7 tooth cog and want a 16 inch wheel to go at 260 rpm how do I work out the correct size of cog to fit to the wheel?

Footie: I see that you are also interested in making a trailer by modifying a tag-a-long. I live in Newquay which is not far from you. Perhaps we could co-operate. I have e-mailed you my telephone number if interested.